The present invention relates to mass spectrometer analysis systems and a laser system for use therein.
Matrix assisted desorption and ionization systems have been developed to desorb and ionize large molecules and with the aid of a mass spectrometer to determine the molecular weight of large molecules e.g. DNA fragments, proteins or other large molecules. See: U.S. Pat. No. RE 37,485 and U.S. Pat. No. 5,288,644. Such devices generally require the introduction of a sample into a target area (sample chamber) that is kept under vacuum. A laser beam is then directed at the target to desorb and ionize the sample.
A problem has arisen with the use of the laser in such systems. The laser beam in such systems must have its spot size adjusted from time to time. As shown in FIG. 4, prior art matrix assisted desorption systems have included a mass spectrometer, a sample chamber for receiving one or more samples under vacuum and a focusing tens. Prior art focusing lenses 72 have been attached to the mass spectrometer by a plurality of rods 74 extending from the mass spectrometer. A laser 70, an attenuator 76, and a folding mirror 78 are mounted to a horizontal plate and aligned. The focusing lens 72 is located in a different plane than the horizontal plate. The plate is mounted to the mass spectrometer 80 by a mounting rod that extends at an acute angle from the mass spectrometer. The laser beam is reflected by the folding mirror 78 to the focusing lens 74 located adjacent the sample chamber.
In such prior art system, adjustment of the laser spot size has been problematical. The focus lens had to be located close to the sample and have a small focal length about 150 mm to 175 mm because the spot size has to be relatively small and adjustable from about 50 xcexcm to about 200 xcexcm in diameter. It proved difficult to adjust the spot size and maintain a predetermined strike point because when the focusing lens was moved, the strike point moved.
The spot size was adjusted by moving the focusing lens along the rods. However when such adjustment was made, the strike point of the laser beam moved on the sample. This resulted in having to adjust the angle of reflection of the folding mirror or move the location of the sample. However locating where the laser beam would strike the sample chamber was difficult because the chamber was under a vacuum and was inaccessible. A camera was used to locate the striking point, but it had a limited spatial coverage.
According to the invention, a matrix assisted laser desorption and ionization system is provided. The system includes a mass spectrometer for analyzing at least one sample. The mass spectrometer includes a sample receiving chamber for receiving a sample under a vacuum. A UV laser is provided for producing a UV beam along a first beam path that does not intersect with the sample A beam expander is provided in optical communication with the UV laser beam to expand the diameter of the beam. A focusing system, desirably a focusing lens, is provided in optical communication with the beam expander to focus the laser beam propagating from the beam expander to a predetermined minimum spot size. The focusing system is located along the path of the beam propagating from the beam expander so that the beam strikes the approximate optical center of the focusing system. A high reflecting mirror is provided in optical communication with the focused beam propagating from the focusing system to direct the focused beam to strike the sample at a preselected strike point. Desirably, the laser, the beam expander and the focusing system, preferably a lens, are mounted to the same mounting surface or to parallel mounting surfaces that are joined together to form an integral structure.
The focusing system is movably mounted desirably along the mounting surface or a parallel mounting surface so that the spot size of the beam at the desired strike point on the sample in the mass spectrometer can be varied. Preferably, the focusing system is mounted on a translation table which allows for easy and accurate movement of the focusing system to adjust the spot size as required. Desirably, the high reflecting mirror is mounted to the same mounting surfaces or to a parallel mounting surface joined together to form an integral structure as are the laser, beam expander and the focusing system. The resulting laser, focusing system and beam expander and optionally the high reflecting mirror form an integral optical module having a unitary structure which can be easily connected to the mass spectrometer after the laser and the associated components have been aligned. When the spot size is changed, there is no substantial change in the strike point on the sample in the sample chamber.
It is an object of the invention to provide a matrix assisted desorption ionization system having a UV laser for producing a beam having a spot size that is adjustable without substantially moving the strike point on the sample chamber of the system.
It is an object of the invention to provide a matrix assisted desorption ionization system having a UV laser for producing a beam having a spot size that is easily adjustable.
It is an object of the invention to provide a matrix assisted desorption ionization system having a UV laser and optic components forming a modular structure which can be aligned prior to attachment to the matrix system.
The preferred embodiment of the present invention is illustrated in the drawings and examples. However, it should be expressly, understood that the present invention should not be limited solely to the illustrative embodiment.